After a long-term operation of the air separation plant, the components in the cryogenic equipment and pipelines in the distillation system will be blocked due to ice or mechanical powder, which will increase the resistance of the fractionator. When the air separation plant has been in normal operation for about 2 years, the fractionator system should be heated and thawed to remove these deposits. When the molecular sieve adsorber and the main heat exchanger have water inflow or other faults, it is also necessary to stop and heat up. In general, the following methods will be used for heating and thawing.
1.Heating and thawing operation during normal shutdown:
After the normal shutdown of the air separation plant, large-scale liquid drainage should be carried out. After the liquid discharge, the equipment should stand for 24 hours, start the air compressor, precooling system and purification system, introduce the dry air into the lower column through the main heat exchanger, and open all liquid discharge valves of the lower column, so that the impurities accumulated in the main heat exchanger and the lower column are reheated and discharged out of the cold box after being heated. Open the liquid air section to flow into the upper column valve V1 and the liquid nitrogen section to flow into the upper column valve V3, introduce the heating gas into the upper column, and open the upper column purge valve according to the flow path for heating. When the inlet and outlet temperatures of the heated gas are basically the same, the heating is completed.
2.Heating and thawing operation during fault shutdown
a.When the air separation plantis shut down due to water inflow into the molecular sieve adsorber: firstly, judge the water inflow. When a small amount of water enters the molecular sieve adsorber, resulting in the reduction of carbon dioxide adsorption layer and the increase of carbon dioxide content entering the main heat exchanger and rectification column, the molecular sieve can be activated to restore the adsorption effect. If the amount of water entering the molecular sieve is too large, resulting in the pulverization of the molecular sieve, the molecular sieve must be replaced. When heating the distillation column, the principle of first lowering the columnand then going up the column must be adhered to. After the lower column is heated completely, the heating gas will be introduced into the upper column and main cooling through V1, V3 valves and the main cooling liquid nitrogen returning to the lower column valve.
b.In case of water inlet failure of the main heat exchanger, the main heat exchanger can adopt two ways: forward heating or reverse heating. Forward heating means that the heating gas enters the main heat exchanger after passing through the normal channel from the molecular sieve adsorber, and is discharged from the blow off valve of the lower column; the reverse heating refers to that the heating gas introduced from the outside enters into the lower column from each blow off valve, and is discharged from the molecular sieve adsorber after back blowing the main heat exchanger. Normal comprehensive heating can not be adopted for heating operation. It is necessary to lower the column first and then go up the column. Close the V1 and V3 valves and return the main cooling liquid nitrogen to the lower column valve, blow out the water from the main heat exchanger through the lower column, and then the gas from the lower column enters the upper column for heating. The booster pipeline in the expander system should also be thoroughly heated and blown out.
